Crystallization and structural relaxation of Co48Mn20Ge10B10Si12 amorphous alloy

Journal of Alloys and Compounds 413 (2006) 206–210

Crystallization and structural relaxation of Co48Mn20Ge10B10Si12 amorphous alloy Suk-Won Hwang, Dong H. Im, I.S. Chun, C.S. Yoon ∗ , C.K. Kim Division of Advanced Materials Science, Hanyang University, 17 Haengdang-Dong, Seongdong-Ku, Seoul 133-791, South Korea Received 28 December 2004; accepted 30 March 2005 Available online 22 August 2005

Abstract Co–Mn–Ge metallic glass was produced by melt-spinning process. When annealed below its crystallization temperature, the alloy exhibited a dramatic increase in magnetization, which was attributed to the relaxation of the amorphous structure. The increased glass stability due to the relaxation by moderate annealing was well demonstrated as the amorphous alloy remained nearly amorphous during the second annealing step even though the annealing temperature was raised well above the crystallization temperature. It was concluded that a redistribution of Ge atoms and subsequent change in chemical short-range ordering may have been responsible for the observed relaxation phenomenon. © 2005 Elsevier B.V. All rights reserved. Keywords: Amorphous materials; Metals; Magnetic measurements

1. Introduction Since its first discovery by Duwez and co-workers [1], metallic glasses formed its own class of new materials with unusual physical properties. Especially, transition metal–metalloid amorphous magnetic alloys containing Fe, Ni, and Co were intensively studied due to their soft magnetic properties such as low coercivities, low hysteresis loss, and high permeabilities combined with excellent mechanical properties [2]. In our previous work [3,4], we introduced Mn into Co-rich amorphous alloy in order to study the influence of structural disorder on the magnetic state and crystallization behavior of the Co–Mn amorphous alloys. In the study, during crystallization of Co78−x Mnx B10 Si12 , we observed formation of Co2 MnSi, which is one of Heusler alloys that has been extensively studied due to its predicted half-metallic property with 100% spin polarization [5]. However, the formation of Co2 MnSi was immediately followed by nucleation of crystalline Co as annealing temperature was increased [3]. In the present work, to form a large fraction of Co2 MnSi crystallites embedded in a paramagnetic amorphous matrix, ∗

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a fraction of Co in the Co–Mn amorphous alloy was replaced by Ge, such replacement making possible the nucleation of Co2 MnGe or solid solution of Co2 MnSi and Co2 MnGe, and at the same time, suppressing the nucleation of crystalline Co. While analyzing the crystallization behavior of the Co48 Mn20 Ge10 B10 Si12 metallic glass, it was found that the amorphous alloy, unlike other Co–Mn amorphous alloys, displayed a pronounced structural relaxation, which irreversibly altered the glass structure as well as its magnetic properties. Here, we report the pronounced structural relaxation of the amorphous Co–Mn–Ge amorphous alloy as well as its crystallization behavior. 2. Experimental details Amorphous Co48 Mn20 Ge10 B10 Si12 alloy strips were produced using the melt-spinning process. Typical samples produced were 20 ␮m thick and 2 mm wide. Composition of the samples was verified with energy-dispersive X-ray spectroscopy and inductively-coupled plasma mass-spectroscopy. Samples were annealed under vacuum (10−5 Torr) for 30 min at temperatures ranging from 200 to 700 ◦ C. Samples for transmission electron microscopy (TEM) were prepared

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using an ion mill equipped liquid-nitrogen cooled cold stage to ensure minimal heating of the sample during milling. The use of liquid-nitrogen cooled cold stage is essential for this alloy, because we found that the heat generated during ionmilling was sufficient to alter the final crystallization product. The electron microscopy was performed using JEM2000EX and JEM2010F (JEOL, Japan). Powder X-ray diffractometer (XRD) (Rigaku, Rint-2000) using Cu K␣ radiation was used to identify the crystalline phase in the material and magnetic hysteresis loops were measured using a vibrating sample magnetometer (VSM) at RT.

3. Results and discussion Fig. 1 shows the in situ magnetic magnetization measurement of the Co48 Mn20 Ge10 B10 Si12 alloy as a function of the annealing temperature. The annealing temperature was raised well beyond the crystallization temperature while maintaining an applied field of 5 kOe and a heating rate of 7 ◦ C/min. The amorphous alloy, which was weakly ferromagnetic in the as-cast state, exhibited a marked rise in magnetization due to the nucleation of Co-enriched ferromagnetic phases at ∼520 ◦ C. Devitrification of the amorphous matrix continued to proceed up to 570 ◦ C, at which the matrix appears to be fully crystallized and then the magnetization markedly fell off. A shoulder around 600 ◦ C suggests formation of another set of ferromagnetic phases as the residual amorphous matrix became increasingly enriched by the solute rejection from the crystalline phases, which would eventually lead to a eutectic reaction of the matrix. The XRD measurements at room temperature (Fig. 2) identify the crystalline phases formed at different temperatures. The initial crystalline phase observed at 550 ◦ C was cubic Co2 Mn(Si,Ge) ˚ which was slightly larger than that of pure with a = 5.710 A, ˚ JCPDS 30-0447) and smaller than Co2 MnSi (a = 5.670 A, ˚ JCPDS 27-1112) due to that of pure Co2 MnGe (a = 5.743 A, probable incorporation of Ge into Co2 MnSi. As the tempera-

Fig. 1. In situ measurement of magnetization at 5 kOe and heating rate of 7 ◦ C/min for as-cast Co48 Mn20 Ge10 B10 Si12 .

Fig. 2. XRD data for Co48 Mn20 Ge10 B10 Si12 annealed at different temperatures.

ture was increased, nucleation and growth of Co2 Mn(Si,Ge) were accompanied by the formation of both hcp and fcc Co phases as can be seen from the XRD data obtained from the sample annealed at 570 ◦ C (Fig. 2). Judging from the relative intensity of the Co and Co2 Mn(Si,Ge) XRD peaks, there exists a substantial amount of crystalline Co phases at this annealing temperature. In fact, when annealed at 600 ◦ C, even though Co2 Mn(Si,Ge) was first to nucleate in the matrix, Co became the dominant crystalline phase. At 570 ◦ C, we also identified Mn5 Ge3 and Co2 Si in the matrix whose nucleation was probably triggered by the formation of crystalline Co. The XRD pattern from the specimen annealed at 600 ◦ C shows that the matrix underwent eutectic transformation, resulting in formation of Co3 B which is weakly ferromagnetic [6], as suggested by the in situ magnetic measurement in Fig. 1. We have previously observed that when amorphous alloys containing similar amounts of Co and Mn were annealed, in some cases, the Co2 MnSi phase formed at the incipient stage of crystallization was metastable and decomposed at higher annealing temperatures [3]. The Co2 Mn(Si,Ge) phase in this alloy was stable so that there was a substantial amount of Co2 Mn(Si,Ge) even after full devitrification. The bright field TEM image and the (1 0 0) zone electron diffraction pattern from a single crystallite from the sample annealed at 550 ◦ C shown in Fig. 3 affirm that the initial crystalline phase was indeed Co2 Mn(Si,Ge). The crystallites show a typical dendritic growth shape with sizes ranging from 50 nm to 200 nm. At 570 ◦ C, the matrix was fully crystallized as can be seen from Fig. 3c. Grains which are relatively free of defects (with uniform contrast) belong to Co2 Mn(Si,Ge), as confirmed by dark-field imaging. Since Co2 Mn(Si,Ge) was to first nucleate in the amorphous matrix, Co2 Mn(Si,Ge) grains tended to be comparatively larger and free of structural defects. From Figs. 1–3, it can be concluded that, like other Co-rich amorphous alloys [3,7,8], Co48 Mn20 Ge10 B10 Si12 devitrified through a primary crystallization, in which metal-rich crys-

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Fig. 3. (a) TEM bright field image; (b) indexed (1 0 0) zone electron diffraction pattern from a single crystallite of the Co48 Mn20 Ge10 B10 Si12 alloy annealed at 550 ◦ C; and (c) TEM bright field image annealed at 570 ◦ C.

talline phases first nucleated, followed by eutectic reaction triggered by the metalloid enrichment of the matrix [9]. This amorphous alloy, however, unlike other Co-rich compositions, showed a substantial irreversible change in magnetic properties when annealed below the crystallization temperature. Differential scanning calorimetry of the amorphous alloy showed a broad peak below 500 ◦ C prior to the nucleation of crystalline phase although the matrix still remained amorphous as can be seen from the XRD pattern in Fig. 4 taken after annealing of the alloy at 400 ◦ C. Because metallic glasses are in a supercooled state, annealing of the amorphous alloys at a temperature where rapid crystallization is inhibited allows the amorphous structure to relax into a more stable amorphous structure [10]. Structural relaxation of the amorphous structure is quite common and changes in mechanical, electrical, and magnetic properties of the amorphous

Fig. 4. XRD data for Co48 Mn20 Ge10 B10 Si12 annealed at 400 ◦ C.

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Fig. 5. Room-temperature magnetic hystersis loops for Co48 Mn20 Ge10 B10 Si12 annealed at different temperatures.

have been used to indirectly verify the relaxation process [9–14]. Fig. 5 shows the magnetic hysteresis loops obtained using VSM at room temperature after annealing the amorphous alloy between 200 and 400 ◦ C. The magnetic susceptibility of the amorphous matrix was dramatically increased by the annealing. The as-cast alloy was weakly ferromagnetic, as can be seen from the inset of Fig. 5 with magnetization of only 0.044 emu/g at an applied field of 2000 Oe wheras the magnetization of the sample annealed at 400 ◦ C reached 2.18 emu/g. The hysteresis loops at different annealing temperatures reflect presence of ferromagnetic phases in the matrix, whose fraction increased proportionally with increasing annealing temperature. Evidently, these changes did not result from formation of new crystalline phases as the matrix remained amorphous as attested by the XRD pattern shown in Fig. 4. None of the TEM analysis of the annealed samples shows any evidence of crystallites formed in the amorphous alloy when annealed up to 400 ◦ C. It has been previously reported that mean magnetic moment of the constituent atoms in Fe–Ni–B and Fe81.4 B18.6 amorphous alloys increased by annealing or by slow quench rate [11,15]. However, the observed increase in magnetic moment for those alloys was minimal (∼10% increase), as compared with the alloy shown here. It is also noted that Co–Mn alloys fabricated without Ge did not show any evidence of structural relaxation prior to devitrification [3]. Structural relaxation of the amorphous structure can arise from both topological relaxation and chemical short-range ordering [9,10]. The topological relaxation normally leads to change in density and thermal expansion; however, it is unlikely that the topological relaxation alone can give rise to such drastic increase in magnetization. Hence, it is conjectured that the magnetic ordering observed in Fig. 5 is likely due to the chemical short-range ordering, involving migration of Ge atoms. It is possible that initially Ge atoms may have been segregated into clusters. During annealing, clustered Ge atoms may have been redistributed uniformly to increase the fraction of Mn Ge bonds, which is reasonable

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since the first crystalline phase formed in the matrix was Co2 Mn(Si,Ge) which requires Mn Ge bonds. The increased fraction of Mn Ge bonds should decrease the fraction of Mn Mn bonds, which is antiferromagnetic when Mn Mn bond distance is sufficiently close [16]. At the same time, the increased fraction of Mn Ge bonds should raise the magnetization of the amorphous alloy as theoretical calculation of alternating thin film stacks of Mn and Ge suggested that Mn–Ge interaction is ferromagnetic [6] and Mn–Ge system is one of well-studied dilute magnetic semiconductor systems [17–19]. Annealing below the crystallization temperature not only altered the magnetic structure, but also irreversibly changed the amorphous structure, which is most strikingly demonstrated by the in situ magnetic measurements taken after annealing the alloy at 200–400 ◦ C shown in Fig. 6a. Peak magnetization of the alloy at 570 ◦ C dropped sharply when the alloy was annealed prior to the in situ experiment. The sample annealed at 400 ◦ C hardly crystallized during the second annealing even though the annealing temperature exceeded 600 ◦ C. From the peak magnetization values from the in situ measurements, the volume fraction of the crystalline phase in the matrix at the end of the in situ experiment

Fig. 6. (a) In situ measurement of magnetization at 5 kOe for Co48 Mn20 Ge10 B10 Si12 pre-annealed below the crystallization temperature and (b) volume fraction of crystalline phase in Co48 Mn20 Ge10 B10 Si12 as a function of annealing temperature.

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tribution of the Ge atoms during annealing. Further investigation is underway to probe magnetic moment of each constituent atoms (Co, Mn, and Ge) using X-ray magnetic circular dichroism analysis before and after the annealing, which should provide a better understanding of the relaxation process. Acknowledgment This work was supported by the Ministry of Science and Technology through the Nanoscopia Center of Excellence at Hanyang University. References

Fig. 7. TEM bright field image of the Co48 Mn20 Ge10 B10 Si12 annealed in two steps: 400 ◦ C/600 ◦ C for 30 min. (The inset shows the diffuse diffraction pattern after two-step annealing.)

was estimated and shown in Fig. 6b. As can be seen from the graph, the volume fraction of the crystalline phase decreased nearly linearly with the initial annealing temperature. Fig. 7 shows the TEM image of amorphous alloy annealed in two steps: 400 ◦ C for 30 min followed by 30 min at 600 ◦ C. The matrix, interspersed by ∼20 nm-sized Co2 Mn(Si,Ge) crystallites, mostly remained amorphous as can be seen from the diffuse diffraction pattern shown in the inset. Fig. 7 clearly demonstrates that the glass stability was significantly increased by the moderate annealing through structural relaxation of the amorphous structure so that the pre-annealed alloy failed to fully crystallize.

4. Conclusion We showed pronounced structural relaxation effects of a Co–Mn–Ge metallic glass, whose magnetic properties were drastically altered when the alloy was annealed below its crystallization temperature. The increased glass stability by moderate annealing was well demonstrated as the amorphous alloy remained nearly amorphous during the second annealing step even though the annealing temperature was raised well above the crystallization temperature. We attributed the observed structural relaxation to redis-

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